1. Field of Invention
The present invention relates to a modulation method for generating modulation signals and, in particular, to a type dependency modulation method that utilizes type-control code words to increase the DC component control ability.
2. Related Art
The common modulation method used in recording media refers to the method that generates code word sequence from data words to be stored in recording media by using specific modulation rules. This cause that data words could to be recording more efficiently for recording media, and be reproduced from code words which recording in recording media using the corresponding demodulation method.
Normally under the Run-length limited (RLL) modulation method, there is a (d, k)-constraint. In the so-called (d, k)-constraint, d refers to the minimal number of logic 0's between two consecutive logic 1's; and k refers to the maximal number of logic 0's between two consecutive logic 1's. Taking the eight-to-fourteen modulation (EFM) modulation method as an example, it can use the 8/14 modulation table to perform the encoding procedure to encode the data words under the (d, k)=(2, 10) constraint. However, in order for any two code words to be connected and satisfy the (d, k)-constraint and to effectively increase the DC component control ability, a merging code of 3-bit long is further inserted into an originally 14-bit code word. Therefore the coding rate becomes 8/17. This does not only greatly decrease the coding rate, but also increasing the complication of encoding algorithm. Another EFMPlus modulation method mainly combines the type and state to provide a modulation method with an 8/16 coding rate under the (d, k)=(2, 10) constraint. However, many code words are used twice in this method. That is, different data words may be encoded into the same code word. Therefore, additional encoding rules are required to avoid errors during decoding procedure.
After the above-mentioned modulation methods, there are some another type dependency modulation methods that combine the type and set concepts are developed. For high coding rate reason, there is relatively fewer code words can be used. So the traditional dependency modulation methods always make a modulation table with repeated code words according to the type dependency rule between two consecutive code words. Although this method can largely increase the overall coding rate, the selectable code words in the encoding procedure are even fewer, resulting in the decrease of the DC component control ability. For most of modulation designs, increasing the length of look-ahead path can actually boost the DC component control ability. However, this has only limited effects and makes the algorithm a lot more complicated than before.
Therefore, how to increase the number of selectable code words in existing type dependency modulation methods in order to increase the DC component control ability is an important subject in the field.